Device for continuous positive airway pressure breathing (CPAP)

Abstract: Apparatus and methods for the automated stop-start control in the administration of continuous positive airway pressure (CPAP) treatment are disclosed. Apparatus for the administration of CPAP treatment includes a controllable flow generator coupled to an air delivery circuit, in turn coupled to a mask for the supply of CPAP to a patient's airways. The starting of the administration of CPAP treatment occurs when it is determined that the patient is wearing the mask. Conversely, the ceasing of the administration of CPAP treatment is done when it is determined that the patient is no longer wearing the mask. The determination of whether the mask is being worn or not can be made in a number of ways. In one embodiment, a pressure transducer (40) derives an electrical signal representative of mask pressure (P₁(t)). If the pressure signal is in excess of the threshold, this is taken as indicative of flow in the air delivery circuit due to respiration, and thus it is determined the mask is being worn. The bistable device (50) then causes the commencement of the administration of CPAP treatment. Current supplied to the flow turbine (4) is sensed by a resistor (59), and the sensed voltage is compared against the threshold by a comparator (62) in order to determine whether there is an unexpected increase in motor current indicative of the mask no longer being worn. In such a case the bistable circuit (50) is reset, causing the administration of CPAP treatment to be ceased.

Description: Field of the Invention This invention relates to automated stop/start control in the administration of continuous positive airway pressure (CPAP) treatment. Background of the Invention The administration of CPAP is common in the treatment of obstructive sleep apnea (OSA) syndrome and upper airway resistance syndrome. The fundamental disclosure of CPAP is made in the specification of International Patent Application No. PCT/AU82/00063, published under No. WO 82/03548. CPAP treatment effectively acts as a pneumatic splint of a patient's upper airways by the provision of a positive air pressure of approximately 10 cm H 2 O, although pressures in the range of approximately 5 - 20 cm H 2 O are encountered. More sophisticated forms of CPAP, such as bi-level CPAP and autosetting CPAP, are described in U.S. Patent No. 5,245,995. Common to all forms of CPAP is a nose, mouth or face mask fitted to a patient having connection via an air delivery tube to an air flow generator. CPAP flow generators are normally switched between a standby or stop mode to a functional mode in which pressurised air or oxygen mix is supplied to the patient by a switch located on the body of, or adjacent to, the flow generator. Alternatively, a cordless radio or infra-red remote control unit can be operated by the patient at some distance from the flow generator. In other cases the control switch (or switches), which may be electrical or pneumatic, are located in the vicinity of the patient's mask, and control signals from them are carried back to the flow generator in parallel with the interconnecting air delivery tubing. As an alternative to a nose or mouth mask, nasal prongs (cannulae) may be inserted into the nares to effect a seal between the air delivery circuit and the patient's airway. In all cases, the patient must manually depress a control button to initiate the CPAP treatment. US 5,245,995 describes a CPAP apparatus having a variable pressure air source and a sound transducer (or pressure transducer) in sound communication with the patient's respiratory system. A feedback system controls the air source, for example in order to increase the output air pressure of the air source in response to, for example, the detection of snoring. W093/09834 describes an auto CPAP system for adaptively providing continuous positive airway pressure in an upper airway system. The auto CPAP system is said to perform detection, analysis, and decision making functions. In this specification the term 'mask' is to be explicitly understood as embracing a nose mask, mouth mask, nose and mouth mask and nasal prongs. Furthermore, reference to a 'mask' being worn indicates sealing engagement with the patient's face or with the entrance to the nares. In particular, the expression 'a mask being worn' embraces the embodiment of nasal prongs being inserted into the nares. There are a number of disadvantages in the known control implementations described above. Firstly, the patient may have to reach out of the bed to press a control on the flow generator. This may be difficult to do if they are already wearing the CPAP mask with its connected tubing, or if the flow generator is some distance from the bed. Secondly, a cordless remote control can be mislaid and its batteries will run down and must be regularly charged or replaced. In some instances too, the remote control must be directed towards the flow generator to function, however the flow generator may be difficult to locate in a dark room. Thirdly, controls incorporated in the mask make it heavier and therefore less comfortable to wear, perhaps leading to lack of compliance with the CPAP treatment. Also, they make cleaning and disassembly more difficult for the patient using CPAP treatment apparatus at home. According to the invention there is provided CPAP apparatus for administering continuous positive airway pressure (CPAP) treatment to a patient, comprising a mask for connection to a patient's airways, a flow generator for supplying air to the mask in order to administer CPAP treatment, an air delivery tube connected between the mask and the flow generator for directing air therebetween, and a controller for commencing and/or stopping administration of CPAP treatment, characterised in that the apparatus further comprises mask detection means for detecting whether the mask is connected to a patient's airways and producing a mask detection signal and transmitting said mask detection signal to said controller, and in that said controller commences and/or stops the administration of CPAP treatment in response to said mask detection signal. Further optional features of the invention are set out in the accompanying dependent claims. Brief Description of the Drawings A number of embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figs. 1a and 1b show arrangements for the determination of air flow rate as an indication of whether a CPAP mask is or is not being worn; Fig. 2 shows an arrangement for the determination of a change in pressure as an indication of whether a CPAP mask is or is not being worn; Figs. 3a, 3b and 3c show arrangements for the determination of the proximity of a mask to a patient's face. Figs. 4a and 4b show further sensor arrangements for the determination of the proximity of a mask to a patient's face; Figs. 5a and 5b show schematic control arrangements in the determination of a CPAP mask not being worn; and Fig. 6 shows a schematic diagram of an embodiment incorporating starting and stopping of CPAP apparatus. Detailed Description of Preferred Embodiments and Best Mode 1. Automatic Start Embodiments of the present invention offer an improvement in controlling a CPAP flow generator by determining that the patient is wearing the CPAP mask (including, as noted above, inserted nasal prongs) and then starting the flow generator automatically, either switching to full treatment pressure or into a pressure ramp controlled by a delay timer. This determination can be achieved in any one or more of four ways. The first such technique is the detection of the flow of air in the breathing (air delivery) circuit caused by the patient breathing. This is achieved by measurement of a change in the pressure drop across a flow restriction located in the breathing circuit, either in the proximity of the mask or the flow generator. Fig. 1a shows a mask 1 interconnected with a flow generator (turbine) 4 by a flexible air delivery tube 2. Part-way along the delivery tube 2 is a flow restriction 3, typically in the nature of a narrowing of the diameter of the air delivery tube 2 or an iris. By measurement of the pressure drop (P 1 - P 2 ) across the flow restriction 3, the presence of air flow can be detected. The pressures P 1 , P 2 are supplied to the CPAP controller 8 by respective interconnecting tubes 13,14. A pressure drop change value of X cm H 2 O, determined by the CPAP controller 8, is set as a threshold and if exceeded is taken as indicative of air flow, and hence that the mask is being worn. Because the sense of the change in pressure drop will be opposed dependent upon inspiration or expiration it is the modulus (or absolute value) of the pressure change which is compared against the threshold. Alternatively, as shown in Fig. 1b, the measurement can be of the change in pressure drop (P 1 - P 2 ) across substantially the whole length of the air delivery tube 2 itself. It is then a simple matter of comparing the modulus of the air flow rate (i.e. pressure drop having been calibrated against air flow rate) as against a threshold value (typically 200 ml/s) to determine that the mask 1 is being worn and that it is appropriate to commence the administration of CPAP treatment. The pressure P 2 also can be the actual flow generator delivery pressure measured at a point internally of the casing that houses the control circuits 8 and the flow generator 4. It will be apparent that the methods described above may continually seek to commence the administration of CPAP treatment when actually being administered due to respiration causing a change in pressure drop. This, of course, presents no problem in the control of the CPAP. Furthermore, the automatic stop embodiments to be described either will override the start command, else the start command would cease being produced. A second method of determining that the mask is being worn is by the detection of a pressure change in the breathing circuit caused by the patient breathing. As shown in Fig. 2, this is achieved by the detection of a change of the pressure P 1 (typically a decrease) in the air delivery tube 2 sensed by a conduit 15 connected with a pressure switch (not shown) located within the controller 8. This measurement is made in or near the mask 1, but equally could be at the flow generator 4, and its absolute value is compared against a threshold of, say, 1 cm H 2 O as an indication of respiration and thus the mask being worn. In the alternative, the time varying pressure signal P 1 (t) can be signal processed to detect the appearance of flow as a slight negative pressure, say, 1 cm H 2 O as the threshold indicative of inspiration, as thus indicative of the CPAP mask being worn. A third method is the detection of the contact with, or close proximity of, the patient's face with the CPAP mask as determined via a change in the electrical characteristics of an electrical circuit having one or more electrodes located in the mask 1. The electrical characteristic can be a measure of impedance or a change in capacitive or the inductive coupling of an electrical circuit connected to the mask. Alternatively, the proximity of the patient's hand can be sensed as that person picks up the mask, as this too will affect the capacitance/inductance of the circuit. As shown in Figs. 3a-3c, this method is achieved by the location in or near the mask 1 of two or more electrodes, shown as a pair of conductive plates 5 in Fig. 3a, capactitive plates 6 in Fig. 3b and an inductive coiled wire 7 in Fig. 3c. Each one of these types of electrode typically is connected to the controller 8 by two interconnecting wires 9. As the mask 1 is brought in proximity to, or in contact with, the patient's face, the electrical state of the sensing circuit within the controller 8 will change. The detectable change is for the reason that a conductive (albeit high resistance) path is provided by the surface of the patient's face. The change will either be as a change in surface resistance (Fig. 3a), a change in capacitance (Fig. 3b) or a change in inductance (Fig. 3c). A fourth method of determining that the mask is being worn is by the detection of the contact of the mask with the patient's face, achieved via an electrical or pneumatic switch located in the mask. As shown in Fig. 4a, this is achieved by an electric switch 10 (or switches) located in the mask 1 in such a position as to be depressed or deformed when the mask 1 is brought into contact with the patient's face. The change of state of the switch 10 is therefore detectable by the controller circuitry 8, and thereafter commences the administration of CPAP treatment. Alternatively, as shown in Fig. 4b, a sac 11 of gas or liquid is similarly located on the mask 1 such that contact with the face causes distortion of the sac, and the concurrent pressure change or volumetric flow in the connecting tube 12 is detected by a pressure switch or other suitable device within the controller 8, thus again commencing the administration of CPAP treatment. 2. Automatic Stop Embodiments of the present invention offer an improvement in controlling a CPAP flow generator by determining whether the CPAP mask is no longer being worn and then switching the flow generator 4 off automatically. This determination can be achieved in any one or more of three ways. The first method is by the detection of increased flow of air in the breathing circuit as a result of the patient's face no longer blocking the mask air outflow. This is achieved by measurement of a change in pressure drop across a flow restriction located in the breathing circuit, either in the proximity of the mask 1 or of the flow generator 4. In other words, it is essentially the same procedure as applied in the first embodiment of 'automatic start', except that the change in pressure drop is of an absolute value equivalent to a different (higher) flow threshold, say 3 cm H 2 O continuing for 3 seconds or more, is used in the comparison with the measured air flow. The threshold is greater than the maximum flow due to the maximum CPAP treatment pressure. The apparatus of Fig. 1a thus applies as previously described. Alternatively, the measurement can be of the pressure drop along the length of all or part of the air delivery tubing itself, again, similarly as previously described in Fig. 1b. Accordingly, detection of an increase in air flow rate will cause the controller 8 to stop the administration of CPAP treatment. It may also be the case that an alarm is sounded in the event that the mask 1 may have inadvertently come off the patient's face, or other such occurrence. A second method is by detecting the drop in pressure in the breathing circuit in the vicinity of the mask to a low or near-zero value, and certainly below the minimum CPAP treatment pressure, caused by the decreased restriction to flow due to the removal of the patient's face from the mask whilst CPAP treatment is being administered. This is achieved by the detection of a change of pressure, typically a decrease to less than 1 cm H 2 O, in the breathing circuit using a pressure transducer or pressure switch. Most usually this measurement is made in the mask or near the mask in the air delivery circuit. Once again, the procedure utilises apparatus very similar to that shown in Fig. 2. Upon sensing the decrease in pressure in excess of the threshold (again, possibly continuing for 3 seconds or more), the controller 8 will automatically stop the CPAP treatment. A third method is by detecting an increase in the speed or the electrical supply current (power consumption) of the flow generator 4 due to the higher volumetric flow in the breathing circuit where the mask is not in place. Both instances exemplify an uncontrolled change in operation of the flow generator. Where the electric motor driving the CPAP flow generator is regulated in pressure, a sudden loss of back pressure due to a mask being removed will result in an increase in rotational speed of the turbine above the rotational speed for any treatment pressure as the turbine attempts to raise the mask pressure. For the case where the motor is speed regulated, the result of a mask being removed is a change (increase) in motor current below a set point current value in consequence of an increased flow loading on the flow generator. Fig. 5a shows the flow generator 4 as a component motor 20 and turbine 21. The winding(s) of the motor 20 (whether AC or DC) are supplied by a power supply 22, one interconnecting cable of which includes a current sensing resistor 23. The motor 20 receives a control signal 24 supplied from a motor controller 25 that effects speed control, including stopping and starting of CPAP treatment. The motor controller 25 also receives, as a speed regulating feedback signal, an output signal 26 indicative of the motor speed. The voltage appearing across the resistor 23 is directly proportional to current, and this voltage potential is applied to a differential amplifier 27. The output from the differential amplifier 27 passes to an averager/integrator circuit 28 that averages the pseudo-current value, and integrates it over a period of time to produce a non-linear signal now indicative of power consumption of the flow generator motor 20. A threshold generator 29 outputs a threshold value to a comparator 30 that also receives the output of the averager/integrator circuit 28. A comparison then is made between the threshold and the calculated power consumption signal, and if the threshold is exceeded, the mask 1 is determined to have been removed and the comparator 30 outputs a control signal to the motor controller 25 causing the motor 20 to be stopped, thus, in turn, stopping the administration of CPAP treatment. The threshold also may vary as a function of the selected pressure of the flow generator. Alternatively, in flow generators where speed is not regulated, as in the case shown in Fig. 5b, the change in motor speed, typically detected from the switching signal to one winding or by the output of a Hall-effect sensor integral within the motor 20 itself, has a relation with increase in supplied pressure. Fig. 5b shows the motor speed signal 26 also being supplied to a motor speed measuring circuit 31. In a similar way as discussed above, the motor speed signal is compared with a threshold in the comparator 30, and if an increase in motor speed above the threshold is determined to have occurred, then a signal is passed to the motor controller 25 to cause the motor 20 to stop, hence ceasing the administration of CPAP treatment. 3. Automatic Start and Stop Fig. 6 shows an embodiment of CPAP apparatus incorporating the features of both automatic start and automatic stop. Where convenient, like reference numerals have been used for components common with the embodiments previously described. The mask pressure signal P 1 (t) is provided by the tube 13 to a pressure transducer 40. The transducer 40 generates a differential output voltage proportional to the mask pressure. The differential voltages are applied to a buffering amplifier 42, and the output voltage then is filtered by a filter component 44, resulting in a voltage signal, the V P (t) proportional to the mask pressure. This voltage is AC coupled by RC circuit 46 to a comparator 48 to remove any static offset. The comparator 48 then compares the voltage signal against a threshold value, V 1T . If the mask is being worn and there is respiration, then flow occurs along the breathing circuit. This then induces a pressure within the mask, which is reflected in the voltage V 1T , and if the flow value is above some minimum (determined by the threshold P 1 (t)) then it is determined that the mask is being worn, in which case the output of the comparator 48 goes high and causes the set output of a bistable logic circuit 50 similarly to go high and signal the motor ON/OFF control circuit 52 in turn to signal the motor speed control circuit 54 to start the flow generator 4. The output of the motor speed controller 54 is six gating signals supplied to a double bridge inverter circuit 56. Thus the flow generator 4 is an embodiment of a fractional horse power three-winding brushless synchronous machine. The flow generator 4 also is provided with a revolution counter 58, perhaps in the form of a pulse generator, that provides a rotational speed signal feedback to the motor speed controller 54. The motor current line contains a low value resistor, R sense , that generates a voltage, V sense , that is directly proportional to the motor current. The voltage is passed via a low pass filter 60 to a comparator 62, where a comparison is made between the sensed voltage and a further threshold voltage, V 2T , in order to determine when there is an unexpected increase in motor current indicative of the mask no longer being worn. That is, the speed feedback mechanism will operate such that if the mask no longer is being worn, the motor speed controller 54 will control the turbine 4 such that there will be an increase in motor current due to an attempt being made to keep the mask pressure at a constant level. In the case that the motor current level exceeds that of the threshold, V 2T then the output of the comparator 62 goes high, and is then coupled by an RC network 64, functioning as a form of timer in accordance with the time constant RxC. The output of the timer circuit 64 passes via a buffering threshold circuit 66 that discriminates the logic 0 and 1 levels to the reset input of the bistable circuit 50, in turn causing the motor ON/OFF control circuit 52 to signal the motor speed control 54 that the flow generator 4 is to be stopped. An additional application of these principles is the detection and quantification of leaks in the breathing circuit, particularly those between the mask and the patient's face. That is, the motor power consumption or speed variation can give a confident indication, below the threshold values described, of the occurrence of mask leaks and the like. Typically, the quantities vary non-linearly with flow and are passed through linearising circuitry (or software) prior to quantification. The combination of automatic start and stop described above can be used to provide automatic control of a CPAP flow generator, starting the machine whenever the mask is being worn, and stopping it whenever not being worn. This provides advantages for patients who get up from their beds during the night and for patients who, because of age (young or old) or physical or mental incapacity, cannot easily operate manual controls. Numerous alterations and modifications, as would be apparent to one skilled in the art, can be made without departing from the scope of the present claims. All such alterations and modifications are to be considered as within the scope of the invention, embodiments of which have been hereinbefore described.

Claims: CPAP apparatus for administering continuous positive airway pressure (CPAP) treatment to a patient, comprising a mask (1) for connection to a patient's airways, a flow generator (4) for supplying air to the mask (1) in order to administer CPAP treatment, an air delivery tube (2) connected between the mask (1) and the flow generator (4) for directing air therebetween, and a controller (8) for commencing and/or stopping administration of CPAP treatment, characterised in that the apparatus further comprises mask detection means (5, 6, 7, 10, 11, 13, 14, 15, 23, 31, 40, 59) for detecting whether the mask (1) is connected to a patient's airways and producing a mask detection signal and transmitting said mask detection signal to said controller (8), and in that said controller (8) commences and/or stops the administration of CPAP treatment in response to said mask detection signal. A CPAP apparatus as claimed in claim 1, characterised in that if said mask detection means (5, 6, 7, 10, 11, 13, 14, 15, 23, 31, 40, 59) detects that the mask (1) is connected to the patient's airways, said mask detection signal is an on-signal which causes the controller to start the flow generator (4). A CPAP apparatus as claimed in claim 2, characterised in that said mask detection means (5, 6, 7, 10, 11, 13, 14, 15, 23, 31, 40, 59) comprises an air flow detector (13, 14) for detecting air flow within said air delivery tube (2). A CPAP apparatus as claimed in claim 2, characterised in that said mask detection means (5, 6, 7, 10, 11, 13, 14, 15, 23, 31, 40, 59) comprises a pressure change detector (15) for detecting pressure changes within said air delivery tube (2). A CPAP apparatus as claimed in claim 2, characterised in that said mask detection means (5, 6, 7, 10, 11, 13, 14, 15, 23, 31, 40, 59) comprises an electrical circuit (5, 6, 7) mounted on the mask (1), and in that said mask detection means (5, 6, 7, 10, 11, 13, 14, 15, 23, 31, 40, 59) detects a change in the electrical characteristics of said electrical circuit (5, 6, 7) when the electrical circuit is brought into proximity with the patient's face. A CPAP apparatus as claimed in claim 5, characterised in that said electrical circuit comprises a pair of conductive plates (5), a pair of capacitive plates (6), or an inductive coiled wire (7). A CPAP apparatus as claimed in claim 2, characterised in that said mask detection means (5, 6, 7, 10, 11, 13, 14, 15, 23, 31, 40, 59) comprises an electrical or pneumatic switch (10, 11) mounted on the mask (1) and arranged to switch when the mask (1) is brought into contact with the patient's face. A CPAP apparatus as claimed in any preceding claim, characterised in that if said mask detection means (5, 6, 7, 10, 11, 13, 14, 15, 23, 31, 40, 59) detects that the mask (1) is not connected to the patient's airways, said mask detection signal is an off-signal which causes the controller to stop the flow generator (4). A CPAP apparatus as claimed in claim 8, characterised in that the mask detection means (5, 6, 7, 10, 11, 13, 14, 15, 23, 31, 40, 59) comprises an air flow detector (13, 14) for detecting air flow within said air delivery tube (2). A CPAP apparatus as claimed in claim 9, characterised in that said off-signal is only produced by the mask detection means (5, 6, 7, 10, 11, 13, 14, 15, 23, 31, 40, 59) if an increased air flow is detected to continue to be above a threshold value for 3 seconds or more. A CPAP apparatus as claimed in claim 8, characterised in that the mask detection means (5, 6, 7, 10, 11, 13, 14, 15, 23, 31, 40, 59) comprises a pressure change detector (15) for detecting pressure changes within said air delivery tube (2). A CPAP apparatus as claimed in claim 11, characterised in that said off-signal is only produced by the mask detection means (5, 6, 7, 10, 11, 13, 14, 15, 23, 31, 40, 59) if a pressure decrease is detected to continue to be below a threshold value for 3 seconds or more. A CPAP apparatus as claimed in claim 8, characterised in that the mask detection means (5, 6, 7, 10, 11, 13, 14, 15, 23, 31, 40, 59) comprises a flow generator speed detector (31) for detecting changes in the speed of the flow generator (4). A CPAP apparatus as claimed in claim 8, characterised in that the mask detection means (5, 6, 7, 10, 11, 13, 14, 15, 23, 31, 40, 59) comprises a flow generator power detector (23, 27, 28) for detecting changes in the power consumption of the flow generator (4).

Inventors: BRYDON JOHN WILLIAM ERNEST, CALLUAUD MICHEL, BRYDON, JOHN WILLIAM ERNEST, CALLUAUD, MICHEL
Applicants: RESMED LTD, RESMED LIMITED
Application year: 1994
Main classification: A61M16
IPC codes: A61M39, A61M16